This invention relates to a novel radiant electric heater utilizing fused silica particulates for thermal and electrical insulation and more particularly to a novel construction for the device.
Radiant electric heaters intended for top surface cooking appliances now employ "microporous" type colloidal silica insulation which can be molded or otherwise formed to provide the base component in said device when customarily supported in a metal dish. For example, there is disclosed in U.S. Pat. No. 5,302,444 a radiant electric heater having such colloidal silica insulation which is said to have an extremely minute particle size no greater than about 100 nanometers diameter together with limited porosity dictated by the same pore size. The particular radiant heater device therein described further includes a two-part construction for the insulation component with a base layer of the insulation material being physically supported in the metal dish and a separate ring-shaped insulation layer resting atop said base insulation layer. Further description of other customary radiant heater constructions of this type are found in U.S. Pat. No. 5,471,737, U.S. Pat. No. 5,512,731 and U.S. Pat. No. 5,517,002 which all require such insulation in order to avoid overheating during device operation. A still further description for preparation of a satisfactory microporous insulation of this type can be found in U.S. Pat. No. 5,556,689 which includes a requirement for a very high specific surface area of the minute oxide particulates being employed for adequate thermal insulation.
In U.S. Pat. No. 3,737,624 there is described an electric grill employing dissimilar thermal and electrical insulation formed with multiple insulation layers interleaved by aluminum reflecting foil to improve heating efficiency. The resistance heating element in said device is also formed by directly depositing a high resistivity metal alloy on the top insulation layer.
The above referenced colloidal silica insulation in prior art radiant electric heaters also remains relatively expensive due to costs of preparation including needed raw materials, processing requirements and still other factors limiting availability. Thus, a preparation of suitable microporous insulation according to the foregoing references can involve elaborate processing techniques such as gel formation, controlled precipitation and fume deposition in order to control average particle size and limited porosity no greater than 100 nanometers maximum. It becomes desirable, therefore, to replace conventional microporous insulation now being used in radiant electric heaters by means which are far less costly and more readily available as well as enabling the device itself to be constructed in an improved manner. As can be further noted in the aforementioned prior art references, the conventional colloidal silica insulating material being employed is also customarily processed entirely in a dry state producing a molded or compressed solid form having the final physical shape desired in said radiant heater device. It has now been discovered that an improved material composition and physical form for said insulation permits construction of the radiant heater device in a manner more readily adapted to automated manufacture. In doing so, the modified insulating material utilizes fused silica of the type conventionally produced in a rotary electric arc furnace (such as disclosed in U.S. Pat. No. 4,217,462) and thereafter mechanically reduced by conventional means to produce a suitable particle size for utilization in accordance with the present invention. It remains only required with said substituted fused silica insulation to further include metal heat shield means in a manner to be disclosed hereinafter for satisfactory retention of heat within the radiant heater device during its operation.
It is an important object of the present invention, therefore, to provide a radiant electric heater constructed in a novel manner.
It is another object of the present invention to provide a novel radiant electric heater employing fused silica particulates as the thermal and electrical insulation means when combined in the device in a novel manner.
Still another object of the present invention is to provide a novel radiant electric heater employing fused silica particulates as the thermal and electrical insulation means together with a modified device construction which further includes combination with operatively associated heat shield means.
These and still further objects of the present invention will become more apparent upon considering the following detailed description of the present invention.